The present invention relates to a hearing aid that is partly powered by a capacitor having a large capacitance.
Most hearing aids are powered by a replaceable battery. Metal-air cells, such as zinc-air cells, are commonly used to power hearing aids because of their stable output voltage during their operating period. Metal-air cells use air to activate the cell. They are activated when air, in particular oxygen, is allowed to enter the cell. Prior to use, the battery is sealed with a pull-tab that covers one or more small openings that allow air to reach an air-cathode assembly within the cell. To activate the battery, the pull-tab is removed and air is allowed into the battery. The battery is then inserted into the hearing aid. The open-circuit voltage of a fresh zinc-air cell is typically 1.4 Volt. During use, the output voltage of the battery decreases slowly with time until the end of the battery life where the output voltage drops more rapidly. The life or operating period of the battery is the period within which the battery output voltage is greater than the minimum operating voltage of the circuit, the battery supplies. It is measured in ampere hours. In an analog hearing aid, the minimum operating voltage is typically 0.9 Volt.
Prior art hearing aids may have a battery alarm circuit that alerts the use when the output voltage of the battery falls below a certain threshold voltage. Thus, the threshold in an analog hearing aid is typically 0.9 Volt.
The emergence of hearing aids with digital circuitry, e.g. digital signal processors, has increased the demands on batteries used to power hearing aids. For example, the digital circuitry does not operate at a supply voltage below 1.1 Volt. Also the current needed to supply the digital circuitry increases rapidly as a function of increasing output volume of the hearing aid, and transient currents drawn by the digital circuitry tend to be larger and of shorter rise and fall times than transient currents drawn by analog circuitry. This means that users of digital hearing aids experience a shortened battery life and in some cases even an extremely shortened battery life.
It is an object of the present invention to provide a hearing aid having an electronic circuit drawing low transient currents with slow rise and fall times so that a battery powering the hearing aid exhibits a long life, i.e. operating period.
According to the present invention, a hearing aid is provided having a housing holding an input transducer for transforming an acoustic input signal into a first electrical signal, a signal processor means for compensating a hearing deficiency by generation of a second electrical signal based on the first electrical signal, an output transducer for conversion of the second signal into sound, an energy source for supplying energy to at least the signal processor, and at least one capacitor with a large capacitance coupled in parallel with the energy source for transient current supply.
The signal processor may be a digital signal processor.
The capacitance of the at least one capacitor is preferably greater than 1 mF, more preferred greater than 4 mF, even more preferred greater than 10 mF, most preferred greater than 20 mF.
Preferably, the energy source is a metal-air battery, such as a zinc-air battery.
The at least one capacitor may comprise a fast super capacitor, e.g. an electrochemical double layer capacitor with a highly conductive polymeric, proton conductive electrolyte of the type disclosed in Technical Information, Bestcap a new dimension in xe2x80x9cfastxe2x80x9d supercapacitors, Scot Tripp, AVX Ltd, Fleet, UK.
Typically, a hearing aid has a housing holding the input and the output transducer and the signal processor and that is adapted to be worn, i.e. behind the ear, in the ear, or in the ear canal, and wherein the output of the output transducer is led to the eardrum in a way that is well-known in the art of hearing aids.
The terminals of the at least one capacitor is preferably connected across the battery terminals of the hearing aid.
In another embodiment, the terminals of the at least one capacitor is connected to the signal processor in such a way that the distance between the terminals and the power supply terminals of the signal processor is substantially minimized.